Conventionally, there has been known a measuring instrument, for instance, the one typified by a micrometer, a micrometer head or the like, that has a main body provided with a female thread and a spindle provided with a male screw, and measures dimensions etc. of a workpiece by advancing and retracting the spindle along with its screwing rotation. An example of the measuring instrument is disclosed in, for instance, Reference 1: JP S49-80260U, Reference 2: JP S54-130152A.
According to such a measuring instrument, displacement per rotation of the spindle is defined by a screw pitch of the male screw formed on the spindle.
The screw pitch of the male screw formed on the conventional spindle is typically 0.5 mm or 0.635 mm.
However, since the screw pitch of the male screw formed on the spindle is 0.5 mm or 0.635 mm, the spindle must be rotated for many times as an object to be measured is changed because the displacement per rotation of the spindle is small, thus raising a problem on its operational performance.
Here, it is conceivable that a multiple thread screw is applied to the male screw of the spindle for increasing the displacement per rotation of the spindle. For example, a triple thread screw may be employed to triple the displacement per rotation. However, in order to machine the multiple thread screw precisely, a plurality of outside helixes must be formed with accurate phase difference. In the case of the triple thread screw, although three outside helixes for a screw need to be formed with the phase difference by 120 degrees, it is difficult to machine such a multiple thread screw with that phase difference maintained precisely, and machining error may cause measurement error. Additionally, it is also difficult to accurately form many outside helixes with the equal pitch, thereby increasing the machining cost thereof as the number of the outside helixes increasing.